The invention relates to detecting pulses, such as with laser rangefinder receivers and, more particularly, to measuring thresholds of return pulses with a pulse discriminator (PD).
In laser rangefinder systems, the distance to a target is determined by measuring the time interval between when a transmitted pulse is produced by a laser pulse source and when its reflection (return pulse) from a target is detected. Usually, a digital range counter is started at the time t0 when the transmitted pulse is detected and stopped at the time tr when the returned pulse is detected. The pulses are normally detected by means of a photodetector, producing corresponding analog signals representative thereof. These analog signals are then processed electronically to generate command signals to start and stop the digital range counter. The resultant time interval measurement (tr-t0) is indicative of the distance between the laser pulse source and the target.
Due to the shape of the analog signal produced by a laser pulse impinging upon a photodetector (often Gaussian) and finite receiver bandwidth, the pulse""s leading edge exhibits a risetime. If a fixed-threshold comparator were used to detect and separate the pulses from (lower amplitude) noise signals, the actual time at which the return pulse crossed the comparator threshold would be a function of the amplitude of the pulse signal. This would introduce considerable error and uncertainty into laser rangefinder distance measurement.
The effect of a fixed-threshold comparator on the accuracy of range measurement can be illustrated by considering its response to two different return pulses of differing amplitude: a strong return pulse and a weak return pulse. The strong return pulse signal would result in a threshold crossing at a relatively low point (early) along its leading edge, resulting in an relatively early counter stop command, thereby producing a time interval measurement indicative of a relatively shorter distance between the pulse source and the target (range to target). Conversely, a relatively low-amplitude return pulse signal that just barely crosses the comparator threshold would result in a threshold crossing very high (late) on its rising edge, producing a relatively later counter stop command, thereby producing a time interval measurement indicative of a relatively longer range to target than would be produced by a stronger return pulse occurring at the same time.
To overcome the amplitude sensitivity of a fixed-threshold comparator, a type of pulse discriminator (PD) known as a constant fraction discriminator (CFD), has been developed to help ensure that the time at which a pulse""s threshold crossing is detected is substantially independent of pulse amplitude. By using the same CFD circuit to detect both the transmitted pulse and the return pulse (thereby producing both the start and stop commands to the digital range counter), the time interval measurement (tr-t0) is substantially unaffected by any delay in the CFD or amplitude variations in the start or stop pulses.
A CFD operates by monitoring the amplitude of the incoming signal (pulse) and continually adjusting its detection threshold to a fixed (i.e., constant) fraction thereof. This threshold level may be produced by attenuating the incoming signal by a fixed attenuation factor and xe2x80x9cstretchingxe2x80x9d the peak of the attenuated signal (e.g., via a peak-hold or xe2x80x9cpulse-stretchingxe2x80x9d circuit). This threshold level is then compared to a delayed version of the incoming signal (e.g., by subtracting it threshold level and detecting zero crossings). The amount by which the incoming signal is delayed is selected to allow sufficient time for the attenuation and peak xe2x80x9cstretchingxe2x80x9d circuit to xe2x80x9cset upxe2x80x9d a valid and stable threshold level. This approach is substantially,independent of pulse shape.
Simpler CFDs do not employ the xe2x80x9cpulse-stretchingxe2x80x9d aspect of the CFD described above. An example of such a simpler CFD is shown in FIG. 1.
FIG. 1 shows a simple prior-art CFD 100 wherein an input signal 102 is applied to a fixed attenuator 104 and a delay line 108. The attenuator 104 scales down the input signal 102 by a fixed attenuation constant xe2x80x9cKxe2x80x9d to produce an attenuated input signal 106. The delay line 108 delays the input signal 102 by a fixed amount, producing a delay signal 110. The delay signal 110 is inverted (multiplied by xe2x88x921) by an inverter 112, to produce an inverted, delayed signal 114. A summing block 116 adds the attenuated input signal 106 to the inverted, delayed signal 114 to produce a summation signal 118. A comparator 120 compares the summation signal 118 to a xe2x80x9czeroxe2x80x9d level to produce a positive output 122 (xe2x80x9cOUTxe2x80x9d) whenever the summation signal is greater than the zero level.
In effect, the CFD of FIG. 1 uses the attenuated input signal 106 as a comparison threshold against which the delayed input signal 110 is compared. The delay is selected to produce the desired threshold crossing point. (In using this technique, it is desired that the shape of the input pulse is constant.)
Another commonly used prior-art pulse detection technique is to differentiate input pulses from a baseline noise level by considering only input signals above a predetermined minimum noise-rejection threshold (effectively a xe2x80x9csquelchxe2x80x9d level). The xe2x80x9csquelchedxe2x80x9d input signal is then differentiated. Due to the natural properties of differentiation, with the correct differentiation time-constant, the differentiated input signal will cross zero at a point corresponding to the peak of the input pulse. Pulse symmetry between rising and falling edges is desirable for an accurate zero crossing time.
Some of the disadvantages of these prior art techniques are:
a) They are complex, especially when multiple channels are used to extend the dynamic range of the CFD or to allow the use of detector arrays.
b) When multiple return pulses are close together, (e.g., as a result of a target behind a tree, with a signal from both the tree and the target) the second return may interfere with the delayed first return, causing a range error or lack of target discrimination, (i.e., the inability to separate and distinguish between the two return pulses). This is especially problematic when the first return pulse is stronger, or when the trailing edge of the first return pulse is elongated (e.g., due to a sloping first target), or distributed in range (e.g., due to multiple closely-spaced echoes from the leaves of a tree).
c) The simpler techniques are sensitive to the pulse shape.
It is a general object of the present invention to provide an improved technique for discriminating between return pulses and improving range accuracy.
It is a further object of the invention to provide a simpler, less-expensive, lower-power pulse discriminator (PD), suitable for use in arrays and expanded dynamic range requirements.
It is a further object of the invention to provide a PD that will allow the resolution of closely spaced pulses, even when a subsequent pulse is small enough that it only appears as a modulation on the trailing edge of a first pulse.
It is a further object of the invention to provide a PD that is easily expandable in dynamic range.
It is a further object of the invention to provide a technique for ensuring an accurate measurement of pulse timing over a wide dynamic range, or in the presence of multiple pulses.
According to the invention, a non-delayed input signal is provided to a first comparator input and a delayed input signal (the delay is applied by a delay line or equivalent delay circuit) is applied to a second comparator input. An offset voltage is applied between the delayed and non-delayed signals at the comparator inputs to provide a xe2x80x9cbiasxe2x80x9d so that the output of the comparator is xe2x80x9cnormallyxe2x80x9d (when no signal is present) at an xe2x80x9cinactivexe2x80x9d state. Typically, the comparator will be a xe2x80x9cfastxe2x80x9d comparator, suited to comparing high-speed analog signals. The comparator compares the delayed and non-delayed signals by effectively subtracting them from one another (with offset applied) and, by greatly amplifying the result to the point of amplifier saturation, effectively producing a logic signal output. Assuming positive signal sense (pulse input is positive going) and that the non-delayed input signal is connected to an inverting input of the comparator, the output of the comparator will xe2x80x9cnormallyxe2x80x9d be xe2x80x9clowxe2x80x9d or inactive. When an input pulse appears on the input signal, the non-delayed input signal will rise immediately and maintain itself more positive than the delayed input, keeping the comparator output xe2x80x9cinactivexe2x80x9d. As long as the input signal is rising (on the leading edge of the input pulse), the comparator output is maintained xe2x80x9clowxe2x80x9d or inactive. When the non-delayed signal reaches its peak and turns downward, the delayed input signal is still rising and crosses over the first pulse, creating a change of state at the comparator output to a xe2x80x9chighxe2x80x9d or active state. The signal edge resulting from this change of start represents initial detection of an input pulse and is used to start or stop a digital range counter. The time of occurrence of this detection edge is substantially independent of the pulse amplitude. In the event that there is a smaller pulse on the xe2x80x9ctailxe2x80x9d of an input pulse, then a crossover may recur again in a similar manner to mark the presence and timing of the third pulse.
According to an embodiment of the invention, a pulse discriminator has an input and an output, and comprises: a first delay line (DL1), a first resistor (R4), a first capacitor (C1), a first adjustable current source (G1), and a first comparator (A2) having two inputs and an output wherein, an input signal (IN) is provided to the input of the first delay line, the output of the first delay line is connected to a first input of the first comparator, the first resistor is connected in parallel across the first capacitor, and is connected between the input of the first delay line and a second input of the first comparator, the adjustable current source is connected to the second input of the first comparator, and the output of the first comparator is the output of the pulse discriminator.
Many signals have a fast rise time and a slower fall time, for example, due to minority carrier diffusion in a photodetector, or decays in the measured event. The present invention is useful for detecting and separating pulses even when these pulses are so closely spaced that their waveforms overlap somewhat.
The dynamic range of the pulse discriminator is limited at the high end by the practical voltage limitations of the fast components needed, and by the offset and noise errors at low signal levels. In order to provide a pulse discriminator with wider dynamic range, additional, parallel pulse discriminators (PDs) of the same type (i.e., delay line, offset, comparator) can be employed, each one adapted to handle a specific range of signal levels. The simple PD circuit of the present invention is readily duplicated and adapted to different signal levels by preceding it with a suitable buffer amplifier or attenuator to scale the input signal up or down. The outputs each of the separate, parallel PD sections for the different ranges are combined with a conventional OR circuit to give a single edge with minimum error over a wide dynamic range.
At low signal levels, it is desirable to prevent noise or offsets from generating an output. This may be accomplished by using a gated comparator in the PD circuit, with a separate low-level threshold detector to generate a gating signal to enable the comparators only for signals above a threshold level, thereby effectively xe2x80x9csquelchingxe2x80x9d signals below the threshold level. An alternative approach is to add a threshold in series with the delayed signal so that for low levels there is an additional separation between the first pulse and the second pulse prior to a crossing. This latter approach would make the PD more sensitive to pulse shape, but virtually every discriminator is subject to increased errors at very low signal levels in the presence of noise.
There is thus provided various methods and means for detecting signals such as return pulses in a laser rangefinder receiver.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.